Hi, this how to buy A goodybag from your airtime credit.
You must be online to buy your goodybag/gigabag.
Log in to your account and click "Top Up" (near the top of the page)
click "add to basket" for your desired goodybag
tick "pay for your order from your airtime credit" box (as desired)
See here for more http://community.giffgaff.com/t5/Learn-giffgaff-Top-Tips/Buying-a-goodybag-from-existing-credit/m-p/...